[gmx-users] No default LJ-14 types

Zuzana Benkova upolzben at savba.sk
Tue May 12 18:30:10 CEST 2009

Dear GROMACS users,

I want to use GROMOS force field for the polyethylene simulations to compare
the performance of this force field with those generated for alkanes. I have
prepared my own files based on the ffG53a5 files. In itp I used

[ defaults ]

; nbfunc        comb-rule       gen-pairs       fudgeLJ fudgeQQ

  1                           1               no              1.0     1.0


and in rtp  

[ bondedtypes ]

; bonds  angles  dihedrals  impropers

    2       2          1          2


In my topology file I required nrexcl = 3 in moleculetype section. I
included [ pairs ] where all 1-4 pairs were given. 

When I did the same for polyalanine system in which case I have used the
ffG53a5 directly from top library, the simulation run and I got  the 1-4
contribution to the potential energy in log file. However in the case of
polyethylene after preprocessing using grompp I got error 41 messages for
each of 41 pairs of the following kind


ERROR 35 [file C44.itp, line 134]:

  No default LJ-14 types


 From manual I expect that if gen-pairs is no, 1-4 interaction are taken
from pairs section and if yes the 1-4 interaction not presented in pairs
section are generated with corresponding fractions as defined in fudgeLJ and


Previously I removed all the pairs section, simulations run but without 1-4
LJ contribution. I have the same problem with OPLS force field, with which
despite the Ryckaert-Bellemans dihedral potential the 1-4 interaction should
be considered. 


I would be very thankful for help. Thank you in advance




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